Throw-away tip

ABSTRACT

A throw-away tip of which two principal surfaces each comprises a land surface  5  provided along the periphery thereof, a central surface  7  provided on the inside of the land surface  5 , a recess (a chip breaker groove) interposed therebetween, and at least one protrusion  8  that extends from the central surface  7  toward the land surface  5 , wherein height of the land surface is made equal to the height of at least the top surface of the protrusion  8  so that at least the top surface of the protrusion of the central surface and the land surface both serve as contact surface.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a throw-away style cutting tipused for cutting a cast iron and a steel.

[0003] 2. Description of Related Art

[0004] Throw-away style cutting tips have been used for cutting a castiron and a steel. Throw-away tips that have modified rake faceconfigurations have been known, such as those having a chip breakerformed in the rake face so as to protect the rake face from beingdamaged by removed metal chips, and those having a land formed near acutting edge along the periphery of the rake face in order to preventdamage from cutting resistance or the like.

[0005] Throw-away tips having chip breakers and lands formed on bothsides have such an advantage as both sides can be used in cuttingoperations. When such a throw-away tip has the cutting edge on one sideworn out, the throw-away tip is turned over so as to carry out cuttingoperation using the cutting edge on the opposite side. A throw-away tipthat uses the edge on either side by turning over is generally designedsuch that height of the land 25 is lower than a seating surface 27(middle of the rake face) (h>0) as shown in FIG. 13, in order to preventthe cutting edge from making contact with a tool holder and beingdamaged (see, for example, Japanese Unexamined Patent Publication No.8-39306).

[0006] However, according to the throw-away tip described above, heightof a tip-corner 33 that has the greatest effect on attaching (seating)of the throw-away tip 21 becomes lower. Therefore, when the throw-awaytip is used in a cutting operation under conditions involving severeimpact, a cutting edge 24 located at the bottom surface (seatingsurface) of the throw-away tip 21 is lifted. As a result, seating of thethrow-away tip 21 becomes insecure thus leading to vibration of thecorner 33 during cutting operation. This causes the possibility of thecutting edge 24 located at the bottom surface of the throw-away tip 21hitting the seating surface of the tool holder intermittently andresulting in breaking (breaking on the back), and/or chatteringvibration of the throw-away tip which may cause chipping of the cuttingedge 24 engaged in the cutting operation, during cutting.

[0007] To avoid such problems, Japanese Unexamined Patent PublicationNo. 11-277307 discloses a throw-away tip that has so-calledfull-circumference chip breaker comprising the land 25 and a centralsurface 27 having similar configuration to the land 25, wherein the land25 and the central surface 27 are formed at the same height so as tosecure the searing of the throw-away tip 21, prevent breaking on theback and reduce the cutting resistance.

[0008] The throw-away tip 21 of full-circumference chip breaker,however, also has such a problem that, in case the land 25 of the rakeface is damaged due to chipping of the edge 24 or a crater wear duringcutting operation with one side of the throw-away tip, the damaged land25 at a corner portion 33 that has the greatest effect on seating of thethrow-away tip 21 cannot provide satisfactory seating surface when thethrow-away tip 21 is turned over. As a result, stability of seating islowered while undesirable movements of the throw-away tip 21 such aslifting and chattering vibration cannot be suppressed and causing suchproblems as tool damage and/or rough machined surface.

[0009] There is also such a problem that, the longer the land surface onone side of the principal surface, the lower the rigidity and moresusceptible to deflection. Particularly when used in a cutting operationunder conditions involving severe impact, deflection of the land surfacecauses chattering vibration which may result in damage of the cuttingedge. This problem becomes particularly conspicuous in a throw-away tiphaving diamond shape that has longer on one side line of the principalsurface.

[0010] The above-mentioned Japanese Unexamined Patent Publication No.11-277307 describes that cutting resistance is reduced by forming thechip breaker. However, since the chip breaker gradually warps over awide area, chips tend to entangle when cutting a material of which chipsare hard to break such as steel, thus resulting in such a problem thatchips are caught between the cutting tool edge and the workpiece thusdisabling stable cutting operation. Also in the case of theabove-mentioned publication, since the chip breaker groove is wider andaccordingly proportion of the contact surface to the entire seatingsurface is not sufficient, there is a weakness in securely fastening thethrow-away tip.

[0011] Moreover, simply decreasing the width of the chip breaker groovereduces the effect of warping the chips and breaking them. When the chipbreaker is made narrower and deeper, there arises the possibility thatchip flow of discharging the chips from the cutting zone is compromised,resulting in such a problem that chips are caught around the cuttingtool edge or the chip breaker groove.

SUMMARY OF THE INVENTION

[0012] In order to solve the problems described above, the presentinvention provides a land along the periphery of the principal surfaceof the throw-away tip, and at least one protrusion that extends from theflat central surface toward the land. The land and the protrusion areformed at the same height. This configuration makes it possible to causean area around the corner, that has the greatest effect on seating ofthe throw-away tip, to serve as the contact surface that makes contactwith the tool holder, thus stabilizing the seating of the throw-awaytip. As a result, even when the cutting edge on one side is brokenduring cutting operation, the area around the broken cutting edge isprevented from being lifted from the tool holder, and therefore breakageon the back can be prevented from being caused by the vibration duringcutting operation. Moreover, chattering vibration of the throw-away tipcan be prevented from occurring, thus preventing the cutting edgeengaged in the cutting operation from being broken and enabling it toprovide smooth finish of the machined surface.

[0013] Throw-away tip of the present invention has substantially flatplate configuration, wherein two principal surfaces provide rake facesand seating surfaces, side surfaces provide relief faces and theintersects of the principal surfaces and the relief faces formingcutting edges. A land surface is provided along the periphery of each ofthe two principal surfaces, and a central surface is provided on theinside of the land surface with a recess (namely, chip breaker groove orthe like) interposed therebetween. At least one protrusion is providedso as to extend from the central surface toward the land surface. Sincethe land and the protrusion are formed at the same height, at least thetop surface of the protrusion (namely the plane part at the distal endof the protrusion) of the central surface and the land surface bothserve as the contact surfaces. The protrusion is preferably formed in anacute angle at the distal end thereof.

[0014] Further according to the present invention, it is preferable toprovide a linkage portion that connects the protrusion and the landsurface. The linkage portion improves the seating stability duringcutting operation, and also reinforces the land surface. As a result,chattering vibration of the land surface can be prevented from occurringduring cutting operation, thus preventing the cutting edge engaged inthe cutting operation from being broken and enabling it to providesmooth finished surface. The linkage portion is most preferable in caseswhere the principal surface has a polygonal shape such as diamond shape.

[0015] In the present invention, the recess (chip breaker groove) isdimensioned in a range from 0.1 mm to 0.6 mm in depth (h₁), from 0.7 mmto 2 mm in width (w), from 0.3 mm to 1.5 min in width (w₁) from the landsurface side to the bottom of the recess, and from 0.5 mm to 1.7 mm inwidth (w₂) from the bottom of the recess to the central surface.

[0016] The recess (chip breaker groove) is substantially V-shaped suchthat the bottom is a steep slope on the land surface side and is agentle slope on the central surface side. The recess is also formed tohave as small width as possible. This makes it possible to properly warpand break chips and discharge the chips smoothly out of the cuttingzone, while maintaining the stability of seating on the seating surface.Because the chip removing performance is excellent, stable cuttingoperation can be achieved even when cutting tough materials such assteel wherein strong impact is generated, or cutting a compositematerial such as steel and cast iron held together.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1(a) is a plan view schematically showing one embodiment ofthe present invention, and FIG. 1(b) is a schematic sectional view takenalong the line C-C in the same.

[0018]FIG. 2 is an enlarged partial plan view of FIG. 1(a).

[0019]FIG. 3 is a schematic enlarged sectional view of portion A in FIG.1(b).

[0020]FIG. 4 is a schematic enlarged sectional view taken along the lineB-B in FIG. 1(a).

[0021]FIG. 5 is a plan view schematically showing another embodiment ofthe present invention

[0022]FIG. 6(a) is a schematic enlarged sectional view taken along theline X-X in FIG. 5, and FIG. 6(b) is a schematic enlarged sectional viewtaken along the line Y-Y in FIG. 5.

[0023]FIG. 7(a) is a schematic plan view of a throw-away tip withoutchip breaker, FIG 7(b) is a schematic plan view of a throw-away tiphaving an ordinary full-circumference chip breaker, and FIG. 7(c) is aschematic plan view of a throw-away tip having protrusion withoutlinkage portion.

[0024]FIG. 8 is a schematic perspective view showing further anotherembodiment of the present invention.

[0025]FIG. 9 is a schematic sectional view taken along the line Z-Z inFIG. 8.

[0026]FIG. 10 is a plan view of the throw-away tip shown in FIG. 8.

[0027]FIG. 11 is a schematic exploded view showing the throw-away tipshown in FIG. 8 through FIG. 10 being attached to a tool holder.

[0028]FIG. 12 is a plan view showing the shape of the principal surfaceof the throw-away tip of Example III-2.

[0029]FIG. 13(a) is an enlarged sectional view of a key portion of athrow-away tip of the prior art, and FIG. 13(b) is a plan view of thesame.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] One embodiment of a throw-away tip of the present invention willbe described below with reference to FIG. 1(a) and FIG. 1(b). Makingreference to FIG. 1, the throw-away tip 1 is a substantially flat plateof polygonal shape that has principal surface 2 providing rake face andseating surface, and relief faces 3 on the side surfaces. Intersects ofthe principal surfaces 2 and the relief faces 3 form cutting edges 4(cutting edge 4 a that engages in cutting operation and cutting edge 4 blocated on the seating surface). A flat land surface 5 is formed inparallel to the seating surface along the periphery of the principalsurface 2. The central portion of the principal surface 2 of thetrow-away tip 1 has a central surface 7 that is parallel to the seatingsurface and is flat. Provided between the land surface 5 and the centralsurface 7 is a chip breaker groove 6 that serves as a chip breaker.

[0031] At least one provision 8 is provided that extends from thecentral surface 7 toward the land surface 5. The protrusion 8, at leastthe top surface thereof, is formed at the same height as the landsurface 5. This configuration makes it possible to cause the landsurface 5 to serve as the contact surface (seating surface) so as tostabilize the seating of the throw-away tip 1, particularly the contactsurface that makes contact with the tool holder, and prevent the cuttingedge 4 b from making chattering vibration or being broken on the backdue to lifting on the cutting edge 4 b during cutting operation. In casethe throw-away tip 1 is supported on the land surface 5 alone, seatingof the throw-away tip 1 becomes unstable when a cutting load is appliedto the land surface 5. When the throw-away tip 1 is seated on at leastthe top surface of the protrusion 8 in the central surface 7 as well ason the land surface 5, in contrast, cutting load applied on the landsurface 5 can be reduced and the throw-away tip 1 can be seated also onthe outside of the throw-away tip 1. This configuration stabilizes theseating of the throw-away tip 1 and constrains lifting of the cuttingedge 4 b portion.

[0032] In case the land surface 5 is lower than the top surface of theprotrusion 8, the cutting edge 4 b portion is lifted during cuttingoperation, causing intermittent impact on the cutting edge and thecentral surface on the mounted side of the throw-away tip 1, resultingin breakage on back or chipping. In case the land surface 5 is higherthan the top surface of the protrusion 8, on the other hand, seating isstabilized and lifting can be suppressed, but at the same time cuttingload is concentrated on the land surface 5, thus leading to breakage onback or chipping.

[0033] Also because the land surface 5 may be damaged by the wear,chipping or the like during cutting operation, the land surface 5 of thenose 4 does not serve as the contact surface when the throw-away tip 1is turned over and put into use. As a result, when cutting load isapplied to the nose 4, seating is not stable and lifting and/orchattering vibration occurs because the nose 4 does not have contactsurface beneath it. When the protrusion 8 is provided, since the seatingsurface is located near to the land surface 5, stability of seating canbe maintained even when the land surface 5 is damaged.

[0034] In order to make the land surface 5 and the top surface of theprotrusion 8 equal in height, the sintered body having the sinteredsurface without ground, that the land surface 5 and the top surface ofthe protrusion 8 is formed to be a certain height, may be used asthrow-away tip. It, however, is preferable that the throw-away tip issintered with the land surface 5 and the top surface of the protrusion 8being formed to be higher than the desired height, and is ground on theland surface 5 and the top surface of the protrusion 8 at the same time.Effects of the present invention are not lost even when there is adifference of up to 0.05 mm in height between the land surface 5 and thetop surface of the protrusion 8 due to variations in the machiningprocess.

[0035] Seating of the throw-away tip 1 can be improved further byproviding a plurality of protrusions 8 outside the central surface 7 tosuch an extent as the cutting operation will not be affected. Angle α ofthe protrusion 8 at the distal end thereof is preferably an acute angle,particularly within 60° as shown in FIG. 2, since this makes it possibleto angle more protrusions 8 near the land surface 5 of the throw-awaytip 1. These protrusions 8 can be functioned as a chip breaker.

[0036] It is also desirable that one of the protrusions 8 of thethrow-away tip 1 extends toward the corner 13 of the throw-away tip 1,and there is one or more protrusion 8 that extends between two corners13. Since the protrusions 8 that extend toward the corners 13 provideboth seating surface and effects of chip breaking, direction of chipremoval can be made constant and cutting resistance can be reduced.Moreover, stability of seating is improved further by providing one ormore protrusions 8 between two corners 13.

[0037] A ratio (L₂/L₁) of the distance L₂ from the center of thethrow-away tip 1 in the distal end of the protrusion 8 to the distanceL₁ from the center of the throw-away tip 1 to the periphery of thethrow-away tip is preferably in a range from 0.7 to 0.95. Such a valuemakes it possible to maintain the stability of seating of the throw-awaytip 1, especially the seating surface, even when there is a breakage inthe land surface or a difference of height between the land surface 5and the central surface 7 due to variation in the machining process. Incase the ratio is less than 0.7, seating becomes unstable when the landsurface 5 is broken. In case the ratio is higher than 0.95, chip removalproperty is deteriorated and curing resistance becomes large, thustending to generate the chattering vibration and chipping.

[0038] As shown in FIG. 1, a clamp hole 17 is formed at the center ofthe throw-away tip 1. A lever, a screw or the like (not shown) isinserted into the clamp hole 17, and the throw-away tip is fastened bypressing the inner wall of the clamp hole 17 to a holder restrictionsurface side. However, the present invention is not limited in the clampattaching system of the throw-away tip, and the lever-lock attachingsystem using the lever as mentioned above, the screw-clamp attachingsystem using the screw may also be employed. Further, top-clampattaching system using the throw-away tip not having the clamp hole 17may be employed.

[0039] In case width of the land surface 5 is in a range from 0.2 to 0.5mm, it is made possible to improve the strength of the cutting edge,improve the stability of seating the throw-away tip 1 on the seatingsurface (contact surface), increase the strength against breaking on theback and prevent breakage and chipping from occurring by improving thechip removing performance and suppressing the increase in cuttingresistance.

[0040] As shown in FIG. 3, the chip breaker groove 6 formed between theland surface 5 and the central surface 7 consists of three curvedsurfaces (R); a curved surface (R₁) 10 located on the land surface 5side, a curved surface (R₂) 11 that continues to the curved surface R₁10 and is located on the bottom of the chip breaker groove 6, and acurved surface (R₃) 12 that continues to the curved surface R₂ 11 and islocated on the central surface side. This structure makes it possible,in addition to the effect of the chip breaker to reduce the cuttingresistance, to increase the chipping resistance, make the chip removalsmoother so as to prevent breakage due to jamming of chips, and tomaintain the areas of the central surface 7 that provides seating andthe protrusion 8.

[0041] In case that a main purpose is to reduce the cutting resistancein order to process a cast iron, the chip breaker groove 6 is preferablyformed with the curved surface R₁ 10 having radius of curvature in arange from 15 to 17 mm. For the purpose of decreasing the length of thechip breaker groove 6 and stabilize the seating while maintaining theflow of chips, radius of curvature of the curved surface R₂ 11 is in arange from 1 to 2 mm, preferably from 1 to 1.5 mm. For the purpose ofmaking the chip removal smoother and preventing the tool from beingbroken due to jamming of chips, radius of curvature of the curvedsurface R₃ 12 is in a range from 22 to 24 mm.

[0042] While the cutting edge 4 may be a sharp edge, it is preferable toperform processing of C surface (chamfer honing surface or tapersurface) or R surface (round honing surface or curved surface) to thecutting edge 4. Such a configuration increases the strength of thecutting edge 4, and reduces the cutting resistance, and thereby makes itpossible to prevent chipping or breakage from occurring on the cuttingedge 4.

[0043] Also as shown in FIG. 1, ratio of the area of the central surface7 and the land surface 5 (namely the contact surface in the principalsurface 2 is preferably from 50 to 90%. This makes it possible tostabilize the seating. Further, chip removal property is increased, andcutting resistance is lowered. As a result, lifting and chatteringvibration is suppressed, thereby preventing breakage on the back andchipping from occurring.

[0044] It is also preferable that distance (w) between the distal end 8a of the protrusion 8 that is located at the same height as the centralsurface 7 and the land surface 5 is within 0.5 mm and, as shown in FIG.4, height of the distal end 8 a of the protrusion decreases by an anglein a range from 10 to 60° toward the land surface 5. This configurationmakes it possible for the operator to visually check the clearancebetween the land surface 5 and the distal end of the protrusion 8 todecrease as the principal surface is ground, and therefore it is madeeasier to monitor the grinding process, so that variation in flatness inthe principal surface 2 upon machining can be prevented.

[0045] Now another embodiment of the present invention will be describedbelow with reference to FIG. 5 and FIG. 6. As shown in FIG. 5, theprincipal surface of a throw-away tip 1′ has substantially polygonalflat plate shape such as diamond shape. A polygonal principal surface 2′that formed the rake face and the seating surface consists of a corner14 (the corner 14 that cuts into the workpiece is called the nose) andside 15. Side surfaces serve as relief surfaces 3′ (see FIG. 6).Intersect of the principal surface 2′ and the relief face 3′ constitutesthe cutting edge 4′. A land surface 5′ that is flat and parallel to theseating surface is provided along the periphery of the principal surface2′. A central surface 7′ that is flat and parallel to the seatingsurface is provided at the center of the principal surface 2′ of thethrow-away tip 1′. A chip breaker groove 6′ (recess) is provided betweenthe land surface 5′ and the central surface 7′.

[0046] According to the present invention, at least one protrusion 8′ isprovided to extend over the central surface 7′ toward the land surface5′, while a linkage portion 9 is provided between the land surface 5′and the protrusion 8′ as shown in FIG. 5 and FIG. 6. The linkage portion9 reinforces the land surface 5 and increases the rigidity of thethrow-away tip. Since the protrusion 8 that continues from the center ofthe rake face over the same surface improves the stability of seating ofthe throw-away tip, the land surface can be constrained from makingchattering vibration due to cutting impact and the cutting edge 4 of theland surface 5 can be prevented from being chipped or broken.

[0047] The linkage portion 9 is a flat surface as shown in FIG. 6(b),and has the same height as at least the protrusion 8′. In addition, aportion where the land surface 5′ and the protrusion 8′ are notconnected is provided by linking the protrusion 8′ which is a part ofthe central surface 7′ and the land surface 5′, instead of linking theland surface 5′ and the central surface 7′ over the entire area therebyto reinforce the land surface 5′. This configuration has effects ofsuppressing the cutting performance of the side edge 15 of the principalsurface 2′ from being dulled by the linkage portion 9′, and maintainingthe cutting performance of the side edge 15 at the cutting edge. Aplurality of linkage portions 9 may also be provided so as to furtherincrease the rigidity of the land surface 5′ and further improve thestability of seating the throw-away tip 1′. When the throw-away tip 1′is mounted, it least the protrusion 8′ and the linkage portion 9 of theprincipal surface 2′ serve as the contact surface. This stabilizes theseating of the throw-away tip 1′, suppresses lifting of the cutting edge4′ and chattering vibration and prevents the throw-away tip 1′ frombeing damaged. In order to further stabilize the seating of thethrow-away tip 1′, it is preferable to make the central surface 7′, theland surface 5′, the protrusion 8′ and the linkage portion 9 all toserve as contact surfaces.

[0048] Square throw-away tips called S-type and triangle throw-away tipscalled T type experience less deformation when sintered, and thereforehave better stability of seating, because every corner has the sameinstance from the center of the throw-away tip. In a diamond-shapedthrow-away tip wherein the angle of nose 16 is 80° or less, in contrast,there is a difference in the distance from the center of the throw-awaytip to each corner. As a result, the diamond-shaped throw-away tip isprone to deformation taking place during sintering and consequentlyunstable seating, and has higher possibility of experiencing defectssuch as damaged cutting edge. In the case of D type throw-away tiphaving chip breaker along the entire circumference wherein the angle ofnose 16 is 55°, in particular, in addition to unstable seating, largerside length (a) of the principal surface 2′ results in insufficientrigidity of the land surface 5′, which makes the land surface 5′ morelikely to undergo chattering vibration. Therefore, the effects of thepresent invention are more conspicuously demonstrated in case theprincipal surface 2′ has diamond shape and, particularly the apex angleis 55° or less.

[0049] Width (b) of the land surface 5′ is preferably in a range of 0.2mm≦b≦0.5 mm, in order to increase the strength of the cutting edge,improve the stability of the land surface 5′ and thereby increase theresistance against breakage on the back. This also makes it possible toprevent breakage or chipping through improvement of chip removingperformance and suppression of cutting resistance from increasing.

[0050] Length (a) of one side edge of the throw-away tip 1′ and distance(c) between the nose 16 and the linkage portion 9 preferably satisfy arelationship: 0.25a≦c≦0.75a, in order to prevent the cutting performanceof the throw-away tip 1′ from being dulled and the cutting resistancefrom increasing.

[0051] Length (a) of one side edge of the throw-away tip 1′ and width(d) of the linkage portion 9 preferably satisfy a relationship: 0.5mm≦d≦(a/3), more preferably 0.5 mm≦d≦(a/5), in order to increase thestrength of the linkage portion 9 and improve the cutting performance ofthe cutting edge.

[0052] Now further another embodiment of the present invention will bedescribed below with reference to FIG. 8 through FIG. 11. As shown inFIG. 8, the throw-away tip 1″ is a substantially flat plate of polygonalshape having principal surfaces 2 a, 2 b that form rake faces andseating surfaces, and side surfaces that provide relief faces 3″. Thethrow-away tip has a cutting edge 4″ (cutting edge 4 a that engages incutting operation and cutting edge 4 b located on the seating surface asshown in FIG. 1(b)) in the intersects of the principal surfaces 2 a, 2 band the relief faces 3″. A land surface 5″ is provided at least alongthe periphery of the principal surfaces 2 a, 2 b that form the cuttingedge 4″, and a central surface 7″ is provided on thee inside of the landsurface 5″ so as to interpose a chip breaker groove 6″ between itselfand the land surface 5″. Each of the principal surfaces 2 a, 2 b can beused as the rake face and the seating surface by turning over thethrow-away tip.

[0053] In both principal surfaces 2 a, 2 b, the land surface 5″ and thecentral surface 7″ are formed to the same height so that the landsurface 5″ and the central surface 7″ both serve as the contactsurfaces. With this configuration, the land surface 5, that is locatedalong the periphery where the force of constraint is the weakest,vibration is most likely to occur due to the impact during the cuttingoperation and breakage on the back is likely to occur among all portionsof the seating surface 2 b, serves as the contact surface together withthe central surface 7. As a result, stability of seating of thethrow-away tip 1″ on the seat surface 21 of the tool holder 20 isgreatly improved, as shown in FIG. 11. Therefore, the cutting edge 4 blocated on the seating surface 2 b will not be lifted during cuttingoperation, and breakage on back and chattering vibration can besuppressed.

[0054] In order to make the land surface 5″ and the central surface 7″equal in height, the throw-away tip is sintered with the land surface 5″and the central surface 7″ being formed to be higher than the desiredheight, and is machine on the land surface 5 and the central surface 7″in plane grinding process at the same time. Effect of the presentinvention is not lost even when there is a difference of up to 0.05 mm(ΔH≦0.05 mm) in height between the land surface 5 and the protrusion 8due to variation in the machining process.

[0055] The chip breaker groove 6″ that continues to the cutting edge 4″and the land surface 5″ as shown in FIG. 9 is dimensioned in a rangefrom 0.01 mm to 0.6 mm, particularly from 0.15 mm to 0.3 mm in groovedepth h₁, from 0.7 mm to 2 mm, particularly from 1 mm to 1.8 mm ingroove width w, from 0.3 mm to 1.5 mm, particularly from 0.3 mm to 0.7mm in width w₁ from the land surface side to the bottom of the chipbreaker, and from 0.5 mm to 1.7 mm, particularly from 0.75 mm to 1.5 mmin width w₂ from the bottom of the chip breaker to the central surface.This dimensioning makes it possible to effectively improve the chipremoving performance with very small chip breaker width w₁ suppress thecutting resistance from increasing and maintain the stability ofseating.

[0056] When h₁ is smaller than 0.1 mm, the effect of warping the chipsis lost and the chips extend toward the central surface. When h₁ islarger than 0.6 mm, chips tend to jam and makes chattering vibrationlikely to occur, and the cutting edge tend to receive significant impactand break.

[0057] When the value of w is smaller than 0.7 mm, it is difficult forthe groove to perform the function of chip breaker, with chips passingover the chip breaker groove so as to extend toward the central surfaceand be caught between the cutting edge and the workpiece. When the valueof w is larger than 2 mm, on the other hand, chip breaking becomesinsufficient and, especially when cutting a tough material such assteel, the chips cannot be reliably broken. In addition, stability ofeating on the seating surface 2 b decreases and the area of the centralsurface that serves as the contact surface decrease, thus making theseating unstable.

[0058] When the value of w₁ is smaller than 0.2 mm, the cutting edgereceives large impact and tends to break. When the value of w₁ is largerthan 0.7 mm, the effect of warping the chips is lost. When the value ofw₂ is smaller than 0.75 mm, cutting resistance becomes excessively high,thus resulting in such problems as the cutting edge 2 a tends to breakor experience chattering vibration which causes breakage on the back ofthe cutting edge 4 b.

[0059] The chip breaker groove 6 as mentioned above is a shape suitablefor processing steel especially, and has an excellent chip removalproperty in cutting process of the workpiece that chips tends to extendand twine.

[0060] The chip breaker groove 6 consists of three curves; a first curve10″ having radius of curvature r₁ in a range from 12 mm to 18 mm,particularly from 15 mm to 17 mm, a second curve 11″ having radius ofcurvature r₂ in a range from 0.1 mm to 0.6 mm, particularly from 0.1 mmto 0.3 mm and a third curve 12″ having radius of curvature r₃ notsmaller than 50 mm, that are connected smoothly in this order from theland surface 5 side. The first curve 10″ having radius of curvature r₁has an effect of reducing the cutting resistance. The second curve 11″having radius of curvature r₂ has an effect of efficiency warping thechips without compromising the flow of chips, while decreasing thelength of the chip breaker groove 6 so as to secure the seating. Thethird curve 12″ having radius of curvature r₃ has an effect of smoothlydischarging the chips and preventing the tool from being damaged byjamming chips. As a result, the chip breaker groove 6 increases theratio of the area of the central surface 7 that provides seating, inaddition to the effects of chip breaker to reduce the cutting resistanceso as to suppress the possibility of chipping and allow smooth removalof chips so as to prevent damage due to jamming chips.

[0061] It is preferable that angle α₁ between the chip breaker groove 6″and the land surface 5″ is in a range from 7° to 25°, particularly from9° to 15°, and angle α₂ between the central surface and the land surface5″ is in a range from 4° to 30°, particularly from 4° to 8°, in order toensure smooth removal of chips.

[0062] When the central surface 7″ and the land surface 5″ are connectedat a position 23 which does not serve as the cutting edge 4″ in theperiphery of the principal surfaces 2 a, 2 b, and the linkage portion 9″that serves as the contact surface is provided, the throw-away tip 1 canbe suppressed from deflecting during cutting operation, so that anenhanced effect is provided for suppressing such a problem as thecutting edge 4 b chatters or is lifted from the seating surface 2 b andhits the contact surface 21 of the holder 20.

[0063] As shown in FIG. 8 and FIG. 10, the central surface 7″ is dividedinto a plurality of blocks by separation grooves 18. This configurationcauses portions located near the corner 13″ of the central surface 7″and portions located away from the corner 13″ to shrink with differentratios during sintering process, thus enabling it to make pre-adjustmentfor the deformation during the sintering process.

[0064] At least one protrusion 8″ is provided to extend over the centralsurface 7″ toward the land surface 5″. Even when the throw away tip 1″is attached to a holder 20 having the contact surface 21 that is worn toform a recess 22 due to the use of a conventional throw-away tip that iscontacted only on the central surface over a long period of time, theprotrusion 8″ located near the land surface 5″ serves as the contactsurface so that excessive load is not applied to the land surface 5″that includes the cutting edge 4 a. Also in case the throw-away tip 1″,that has the cutting edge 4 a worn out or chipped and the land surface5″ damaged after using one principal surface 2 a as rake face, is turnedover and mounted but the damaged land surface 5″ does not function asthe contact surface, the protrusion 8″ located near the land surface 5″serves as the contact surface instead of the land surface 5″, andtherefore stable seating can be maintained.

[0065] In the case of such a configuration as the central surface 7″,the protrusion 8″ and the land surface 5″ serve as the contact surfacesas in this embodiment, since the load is distributed over the entirecontact surface 21 of the tool holder 20, no recess 22 will be formed onthe contact surface 21 of the tool holder 20 after extended period ofuse.

[0066] The shape of the distal end portion of the protrusion 8A that isformed from the central surface 7″ toward every corner 13″ may have asharp point with decreasing width formed by concave curves on bothsides, a sharp point with decreasing width formed by straight lines onboth sides, a sharp point with decreasing width formed by convex curveson both sides or a round protrusion. Among these, sharp point withdecreasing width formed by concave curves on both sides is mostpreferable for the reason of achieving both the stability of seating andchip removing performance by adjusting the proper area of the chipbreaker groove 6 and the central surface 7.

[0067] Since one of the protrusions 8″ of the throw-away tip 1″ extendstoward the corner 13″, seating is more stabilized. When the chip breakergroove 6 is provided on the extended line of the protrusions 8 thatextends toward the corner 13″, the direction of removing chips can bemade constant so as to achieve higher effect of chip breaking function,with the cutting resistance reduced further. Moreover, it is preferableto provide at least one protrusion 8″ that extends toward the side edgeother than the corner 13″, for further improving the stability ofseating. Furthermore, since a chip breaker groove 6 a is formed alsoalong the side edge near the corner 13″ that is interposed between theprotrusions 8B, 8B, the side edge near the corner 13 can also be used asthe cutting edge with low cutting resistance and good chip removingperformance.

[0068] The ratio (L₂/L₁) of the distance L₂ from the center of theprincipal surface 2″ to the distal end of the protrusion 8″ to thedistance L₁ from the center of the principal surface 2″ to the peripheryof the principal surface 2″ is preferably not less than 0.7, morepreferably in a range from 0.8 to 0.95, so as to maintain the stabilityof seating and chip removing performance and reduce cutting resistance.

[0069] The ratio of the total area of the central surface 7 and the landsurface 5 in the principal surface 2″, namely the ratio of contactsurface, is preferably from 50 to 90%, more preferably from 80 ti 90%,for stabilizing the seating, improve the chip removing performance, andreduce cutting resistance.

[0070] It is also preferable to set the width of the land surface 5 atthe cutting edge 4″ in a range from 0.2 to 0.5 mm, for increasing thestrength of the cutting edge, improving the stability of seating thethrow away tip 1 at the seating surface (contact surface) and increasingthe resistance against breakage on the back, as well as for preventingbreakage and chipping by improving the chip removing performance andpreventing the cutting resistance from increasing.

[0071] The cutting edge 4″ may be a sharp edge but it is preferable toprovide C surface (chamfer honing surface) or R surface (round honingsurface) to the cutting edge 4″, for the reason of increasing thestrength of the cutting edge and prevent chipping or breakage fromoccurring on the cutting edge 4″.

[0072] Also in FIG. 8, protrusions 8B, that are located near the landsurface 5″ at a distance of 0.5 mm or less, are formed at symmetricalpositions around the plurality of corners 13″, 13″, with height of thedistal end of the protrusion 8B gradually decreasing by an angle in arange from 10 to 60° toward the land surface 5. This configuration makesit possible for the operator to visually check the clearance between theland surface 5 and the distal end of the protrusion 8 to decrease whengrinding the principal surface, and therefore it is made easier to carryout the grinding process, so that variation of flatness in the principalsurface 2 can be prevented.

[0073] As shown in FIG. 10, both the sides 19 and 19 of the protrusion8″ have such a configuration as to slope downward and then rise towardsaid land surface 5. This makes it possible to secure stable and highchip-processing performance, even when the chip discharge direction ischangeable with cutting conditions. Further, even when using as acutting edge the neighborhood part which separated a few from the corner13″, it is possible to suppress chattering vibration by good chipprocessing performance and low cutting resistance, both sides of saidprotrusion.

[0074] The following examples illustrate the manner in which the presentinvention can be practiced. It is understood, however, that the examplesare for the purpose of illustration and the invention is not to beregarded as limited to any of the sp cific materials or conditiontherein.

EXAMPLE I

[0075] A mixed powder prepared by adding 11% by weight of Co to WCpowder was pressed to form green compacts of types and specificationsshown in Table 1 each having two noses on each side, four noses in all.After being sintered in vacuum, samples I-1 through 6 were ground on thecentral surface and on the land surface of each principal surface tomake the height of the central surface and the height of the landsurface equal. In samples I-1 through 6 and 8 through 11, centralsurfaces were formed so as to protrude in symmetrical configurations atopposing corners, while forming a slope of 45° at the distal end andsetting the clearance (w) between the distal end of the projection ofthe central surface and the land surface to 0.2 mm, so that variation inthe parallelism during the grinding process could be monitored visually.

[0076] Sample No. I-7 was formed in the conventional shape without chipbreaker, samples No. I-8, 9 were formed in the conventional shape wherethe land surface is lower than the protrusion, samples No, I-10, 11 wereformed in the conventional shape where the land surface is higher thanthe protrusion, and sample No. I-12 was formed in the conventional shapewhere the land surface has the same height as the central surface andthe central surface does not have protrusion.

[0077] All samples were processed on the periphery of the land surfaceto provide honed edge, and were coated with a hard film ofTiCN—Al₂O₃—TiN successively by CVD process, thereby making thethrow-away tips of No. I-1through 11.

[0078] With the throw-away tips manufactured as described above beingplaced on a flat plate, maximum value of clearance (h) between theperiphery of the land surface and the flat plate, and the position ofthe distal end of the central surface protrusion (mean value f L₂/L₁)were measured. The results are shown in Table 1.

[0079] Cutting tests were conducted under the conditions described belowusing the throw-away tips, while measuring the lift during cuttingoperation and cutting time before breakage of the throwaway tip, andobserving the machined surface of the workpiece. For every sample, thecutting test was conducted with the noses 4 a ₁, 4 a ₂ of FIG. 1(b) and,with the throw-away tip turned over, with the noses 4 b ₁, 4 b ₂. Theresults are shown in Table 2.

[0080] <Cutting conditions>

[0081] Cutting speed: 250 m/min.

[0082] Infeed: 4 mm

[0083] Feed rate: 0.35 mm/rev.

[0084] Workpiece: FC250 with four grooves

[0085] Dry cutting TABLE 1 Sample Ratio of Areas⁽¹⁾ Land Width(b) No.Modol h(mm) R₁(mm) R₂(mm) R₃(mm) L₂/L₁ (%) (mm) I-1 CNMG120408  0   161.2 22.9 0.9 85 0.3 I-2 CNMG120408  0   16 1.2 23.1 0.7 70 0.2 I-3CNMG120408  0   16 1.2 23 0.6 58 0.5 I-4 CNMG120408  0.002 15 1.8 22.30.8 78 0.3 I-5 CNMG120408  −0.003 15.4 1.1 22.1 0.5 52 0.5 I-6CNMG120408  0.008 16.8 1.6 23.5 0.6 67 0.3 * I-7  CNMA120408 — — — 100 — * I-8  CNMG120400  0.2  23 — — 0.95 94 0.2 * I-9  CNMG120408 −0.2  10— 18 0.4 42 0.6 * I-10  CNMG120408  0 15 18 3 25 0.4 38 0.7 * I-11 CNMG120408  0.1  11 1 17 0.35 31 0.1 * I-12  CNMG120408  0   12 1 19 —40 0.2

[0086] TABLE 2 Cutting Edge 4a₁ Cutting Edge 4a₂ Cutting Edge 4b CuttingEdge 4b₂ Sample Cutting Time Lift Machined Cutting Time Lift MachinedCutting Time Lift Machined Cutting Time Lift Machined No. (sec) (μm)Surface (sec) (μm) Surface (sec) (μm) Surface (sec) (μm) Surface I-1 3802 Good 350 3 Good 330 6 Good 340 5 Good I-2 350 5 Good 360 5 Good 320 7Good 350 6 Good I-3 340 6 Good 360 5 Good 330 7 Good 320 8 Good I-4 3505 Good 330 7 Good 310 5 Good 320 8 Good I-5 320 8 Good 310 6 Good 310 6Good 300 7 Good I-6 320 6 Good 300 6 Good 320 8 Good 310 9 Good * I-7 300 3 Rough 290 2 Rough 280 8 Rough 290 8 Rough * I-8  180 65  Rough 17570  Rough 165 83  Rough 165 72  Rough * I-9  180 50  Rough 170 62  Rough165 70  Rough 160 74  Rough * I-10  190 23  Good 180 22  Good 160 42 Rough 150 50  Rough * I-11  200 15  Good 180 15  Good 170 45  Rough 18052  Rough * I-12  280 8 Good 270 9 Good 180 40  Somewhat 175 43 Somewhat Rough Rough

[0087] From Table 2, it can be seen that samples I-1 through 6 where theland surface and at least the protrusion of the central surface wereformed at the same height (h≦0.05 mm) showed lifting of not larger than10 μm at the periphery of the throw-away tip on all cutting edges 4 a ₁,4 a ₂, 4 b ₁, 4 b ₂, long cutting time of 300 seconds or more, nobreakage on back at portions other than the cutting edge, no chipping ofthe cutting edge engaged in machining, and excellent breakageresistance. Observation of the machined surface showed uniformly smoothsurface.

[0088] In the case of sample No. I-7 without chip breaker, the machinedworkpiece showed rough surface.

[0089] Samples No. I-8, 9 having the land surface lower than theprotrusion showed lifting of 50 μm or more and experienced breakage onthe back with the cutting edge being chipped in an early stage, thusshowing low breakage resistance. Smooth machined surface could not beobtained due to chattering vibration.

[0090] Samples No. I-10, 11 having the land surface higher than theprotrusion showed lifting, although in a small extent, and experiencedbreakage of the land surface on the back. As a result, chatteringvibration occurred with the cutting edge being chipped in an earlystage, and the machined workpiece showed unsatisfactorily surface.

[0091] Sample No. I-12, that had the land surface formed in the sameheight as the central surface without any protrusion on the centralsurface, showed no lifting during the cutting test using the cuttingedges 4 a ₁, 4 a ₂, with the cutting time as long as about 300 seconds.However, in the cutting test using the cutting edges 4 b ₁, 4 b ₂conducted after the cutting test of the cutting edges 4 a ₁, 4 a ₂, thelands of the cutting edges 4 a ₁, 4 a ₂ were damaged in the preceedingcutting test and did not serve as the contact surface, thus resulting inlifting and a cutting time shorter than that with the cutting edges 4 a₁, 4 a ₂ while producing somewhat rough machined surface.

EXAMPL II

[0092] A mixed powder prepared by adding 11% by weight of Co to WCpowder was pressed to form green compacts that would make sintering ofthe shapes and dimensions shown in FIGS. 5, 7 and Table 3. After beingsintered in vacuum samples II-1 through 4 were ground on the centralsurface and on the land surface of each principal surface so as to makethe height of the central surface and the height of the land surfaceequal. Samples II-1 through 8 were formed in diamond shape having noseangle θ of 55° with a linkage portion formed (FIG. 5). Sample No. II-5was formed in the conventional shape without chip breaker as shown inFIG. 7(a), sample No. II-6 was formed in such a shape that hasfull-circumference chip breaker while the land surface has the sameheight as the central surface, without protrusion as shown in FIG. 7(b),and samples No. II-7, 8 were formed in a shape without linkage portionas shown in FIG. 7(c).

[0093] All samples were processed on the periphery of the land surfaceto provide honed edge, and were coated with hard films of TiCN—Al₂O₃—TINsuccessively by CVD process, thereby making the throw-away tips of No.II-1 through 8.

[0094] With the throw-away tips manufactured as described above, cuttingtests were conducted under the conditions described below whilemeasuring the cutting time before breakage of the throw-away tip andobserving the machined surface of the workpiece. The results are shownin Table 3.

[0095] <Cutting conditions>

[0096] Cutting speed: 250 m/min.

[0097] Infeed: 4 mm

[0098] Feed rate: 0.35 mm/rev.

[0099] Workpiece: FC250 with four grooves

[0100] Dry cutting

[0101] Samples II-1′ through 7′ were prepared similarly to the samplesNo. II-1 through 6 of Table 3 except for changing the throw-away tiptype to C, and were subjected to cutting test similarly to thatdescribed above. The results are shown in Table 4. TABLE 3 SampleExistence of c b d Height Difference Between Cutting Time DamageMachined No. Model Linkage portion 9 (mm) (mm) (mm) Land andProtrusion(mm) (sec) (Back Breakage) Surface II-1 DNMG150408 Yes 4 0.3 30 380 No Good II-2 DNMG150408 Yes 8 0.2 1.3 0 350 No Good II-3DNMG150408 Yes 4 0.5 1.5 0 340 No Good II-4 DNMG150408 Yes 7 0.3 2 0 350No Good * II-5  DNMA150408 — — — — — 300 Back Breakage Rough * II-6 DNMG150408 No — 0.2 — — 200 Back Breakage Somewhat Rough * II-7 DNMG150408 No — 0.6 — 0 290 No Somewhat Rough * II-8  DNMG150408 No —0.7 —  0.4  50 Back Breakage Somewhat Rough

[0102] TABLE 4 Sample Existence of c b d Height Difference BetweenCutting Time Damage Machined No. Model Linkage portion 9 (mm) (mm) (mm)Land and Protrusion(mm) (sec) (Back Breakage) Surface II-1′ CNMG120408Yes 4 0.3 3 0 540 No Good II-2′ CNMG120408 Yes 8 0.2 1.3 0 520 No GoodII-3′ CNMG120408 Yes 4 0.5 1.5 0 480 No Good II-4′ CNMG120408 Yes 7 0.32 0 500 No Good * II-5′  CNMA120408 — — — — — 410 Back Breakage Rough *II-6′  CNMG120408 No — 0.2 — — 260 Back Breakage Somewhat Rough * II-7′ CNMG120408 No — 0.7 —   0.4 200 Back Breakage Rough

[0103] From Table 3 and Table 4, it can be seen that samples II-1through 4 and samples II-1′ through 4′, where width d of the linkageportion was set in a range from 0.5 mm to a/3, width b of the landsurface was set in a range from 0.2 mm to 0.5 mm while the land surfaceand the protrusion were formed at the same height, showed long cuttingtime of 300 seconds or more, no breakage on back, no chipping of thecutting edge and excellent breakage resistance. Observation of themachined workpiece showed uniformly smooth surface.

[0104] In the tests using samples Nos. II-5, 5′ not provided with chipbreaker, the machined workpiece showed rough surface due to chatteringvibration. Breakage on back also occurred in the contact surface of thethrow-away tips.

[0105] Samples Nos. II-6 through 8, 6′ and 7′ experienced chipping onthe cutting edge in an early stage and showed rough machined surface dueto chipping and chattering vibration. Samples Nos. II-6, 6′, 8 and 7′also experienced breakage on back occurring in the contact surface ofthe throw-away tips.

EXAMPLE III Example III-1

[0106] A mixed powder prepared by adding 11% by weight of Co to WCpowder was pressed to form green compacts of types and dimensions shownin Table 5 having such a pattern as shown in FIG. 8. After beingsintered in vacuum, the central surface and the land surface were ground(double-head grinding) on each principal surface to make the height asshown in Table 1 (samples Nos. III-1 through 10).

[0107] When grinding the throw-away tips, the protrusions 8A located atthe corners 13″ were formed at a distance (w) of 0.2 mm from theprotrusion 8A to the land surface in such a shape as to form a downwardslope of 45° from the protrusion 8A to the land surface, while settingthe clearance (w) between the distal end of the projection and the landsurface being monitored visually when grinding the surface, therebymaking it possible to achieve machining with high accuracy easily. Thesamples were processed on the periphery of the land surface to providehoned edge using a brush, and were coated with hard films of TiCN 2 μmthick, Al₂O₃ 2 μm thick and TiN2 μm thick successively by CVD process,with part of the hard film on the cutting edge slightly polished with awhetstone thereby making the throw-away tips.

Example III-2

[0108] The throw-away tip of the configuration shown in Table 5 (sampleNo. III-11) was made similarly to Example III-1 except for providing thepattern of the principal surface with full-circumference chip breakershown in FIG. 12 to the throw-away tip of the first embodiment.

Comparative Example

[0109] The throw-away tip of the configuration shown in Table 5 (sampleNo. III-12) was made similarly to the Example except for forming theprincipal surface in a single plane without chip breaker on thethrow-away tip of the Example. TABLE 5 Sample ΔH h W W₁ W₂ R₁ R₂ R₃Ratio of Areas¹⁾ Land Width(b) No. Model (mm) (mm) (mm) (mm) (mm) (mm)(mm) (mm) L₂/L₁ (%) (mm) III-1 TNMG120408 0 0.2 1.47 0.53 0.94 16 0.2 ∞0.92 96 0.3 III-2 TNMG120408 0 0.25 1.0 0.4 0.6 14 0.3 100 0.75 78 0.2III-3 TNMG120408 −0.002 0.17 1.43 0.33 1.1 16 0.5 40 0.9 98 0.4 III-4TNMG120408 0 0.2 1.8 0.9 0.9 16 1.2 22.9 0.9 95 0.5 * III-5  TNMG120408−0.2 0.05 3.0 1.0 2.0 16 0.2 ∞ 0.95 94 0.2 * III-6  TNMG120408 0 0.081.47 0.53 0.94 16 0.2 ∞ 0.92 98 0.2 * III-7  TNMG120408 0 0.8 1.47 0.530.94 16 0.2 ∞ 0.92 98 0.2 * III-8  TNMG120408 0 0.2 0.5 0.1 0.75 16 0.2∞ 0.98 95 0.2 * III-9  TNMG120408 0 0.2 2.5 0.7 1.6 16 0.2 ∞ 0.55 580.2 * III-10  TNMG120408 0 0.2 2 1.6 0.4 16 0.2 ∞ 0.6 70 0.2  III-11TNMG120408 0 0.2 1.8 0.8 1.0 12 1.0 19 — 40 0.2 * III-12  TNMA120408 —

[0110] With the throw-away tips manufactured as described above beingplaced on a flat plate, maximum value of clearance ΔH (mm) between theperiphery of the land surface and the flat plate was measured, meanvalue of the distance between the distal end of the protrusion and theland surface (L₂/L₁) was determined using a microscope, and the shape ofthe chip breaker groove was measured, with the result shown in Table 5.

[0111] Cutting tests were conducted under the conditions described belowusing the throw-away tips described above, while measuring the liftduring cutting operation and cutting time before breakage of thethrow-away tip, and observing the number of breakages on back, conditionor chips being removed and the machined surface of the workpiece. Forevery sample, the cutting test was conducted with the three cuttingedges 4 a on the principal surface 2 b shown in FIG. 8 and, with thethrow-away tip turned over, with the three cutting edges 4 b on theopposite surface. The results were averaged and are shown in Table 6.Chip removing performance was evaluated by changing the infeed and thefeed rate within the ranges described below while observing the chipremoving performance over the entire ranges.

[0112] <Cutting conditions>

[0113] Cutting speed: 150 m/min.

[0114] Infeed: 0.7 to 3 mm

[0115] Feed rate: 0.2 to 0.5 mm/rev.

[0116] Workpiece: SCM435

[0117] Wet cutting TABLE 6 Number Chip Cutting Sample of Dack RemovingTime Lift Machined No. Breakages Performance (sec.) (μm) Surface III-1 0◯ 360 0 Good III-2 0 ⊚ 340 5 Good III-3 0 ◯ 340 8 Good III-4 0 Δ 340 2Good *III-5  15 Δ 150 65 Rough *III-6  3 X 240 7 Rough *III-7  0 X 210 2Good *III-8  1 X 200 6 Rough *III-9  0 X 250 2 Good *III-10 0 X 250 3Good  III-11 3 Δ 330 8 Good *III-12 0 X 290 3 Rough

[0118] From Table 6, it can be seen that samples III-1 through 4 and11where the land surface and at least the protrusion of the centralsurface were formed at the same height (h≦0.05 mm) with chip breaker ofthe predetermined configuration showed lifting not larger than 10 μm atthe periphery of the throw-away tip while cutting with any of thecutting edges, and long cutting time of 300 seconds or more. The samplesIII-1 through 4 and 11 showed three or less breakages on back inportions other than the cutting edges, no chipping of the cutting edgeengaged in machining, and good cutting performance in terms of both thesmoothness of the machined surface and chip removing performances. Thesamples III-1 through 4 having the protrusion in the central surface asshown in FIG. 8, in particular, showed no breakage on back at all,without such troubles as unstable seating when the throw-away tip isturned over, and stable cutting performance.

[0119] The sample III-5 where the land surface was lower than theprotrusion, in contrast, showed lifting of 50 μm or more which resultedin lots of breakages on the back. It also experienced chipping of thecutting edge in an early stage, thus showing poor breakage resistance.In addition, smooth machined surface could not be obtained due tochattering vibration.

[0120] In th sample III-12 where chip breaker groove was not formed, inaddition to poor chip removing performance because of the absence ofchip breaker groove, high cutting resistance caused poor cutting, thusresulting in rough machined surface.

[0121] The samples III-6 through 10 having the chip breaker grooves ofother than the predetermined shape showed, though the lifting was small,poor chip removing performance that caused excessively long chips,jamming of chips and/or the chips being caught between the throw-awaytip and the workpiece that forced interruption of the cutting operation.As a result, the cutting edges were broken during cutting and theworkpiece showed poor machined surface.

[0122] In the Examples described above, throw-away tips of type T wereused. In the meantime, samples of type C (CNMG120408) and type D(DNMG150408) having the dimensions of the sample III-1 were made andsubjected to similar cutting test and observations. These samples showedgood chip removing performance, no lifting, no breakage on back, goodmachined surface and long cutting time of 400 seconds and 300 seconds,respectively.

What is claimed is:
 1. A throw-away tip of substantially flat plateshape comprising two principal surfaces, that provide rake faces andseating surfaces, and side surfaces that provide relief faces, withcutting edges formed on the intersects of the principal surfaces and therelief faces so that the throw-away tip can be used on both sides,wherein each of said principal surfaces comprises a land surfaceprovided along the periphery thereof, a central surface provided on theinside of said land surface, a recess interposed between the landsurface and the central surface, and at least one protrusion thatextends from the central surface toward said land surface; and at leastthe top surface of said protrusion and said land surface serve as thecontact surface when said throw-away tip is attached to a tool holder.2. The throw-away tip according to claim 1, wherein the tip surface ofsaid protrusion and said land surface have the same height.
 3. Thethrow-away tip according to claim 1, wherein the top surface of saidprotrusion is acute-angled at the distal end thereof.
 4. The throw-awaytip according to claim 1, wherein one of said protrusions extends towarda corner of said throw-away tip and one or more other protrusion extendsmidway between two corners.
 5. The throw-away tip according to claim 1,wherein the ratio (L₂/L₁) of the distance L₂ from the center of saidthrow-away tip to the distal end of said protrusion to the distance L₁from the center of said throw-away tip to the periphery of thethrow-away tip is in a range from 0.7 to 0.95.
 6. The throw-away tipaccording to claim 1, wherein width of said land surface is in a rangefrom 0.2 to 0.5 mm.
 7. The throw-away tip according to claim 1, whereinsaid recess consists of three curved surfaces: a curved surface (R₁)located on the side of said land surface, a curved surface (R₂) thatcontinues to said curved surface R₁ and a curved surface (R₃) thatcontinues to said curved surface R₂ and is located on the centralsurface side.
 8. The throw-away tip according to claim 1, wherein theratio of areas of said central surface and said land surface in saidprincipal surface is in a range from 50 to 90%.
 9. The throw-away tipaccording to claim 1, wherein the distance between the distal end ofsaid protrusion, that is located at the same height as said centralsurface, and said land surface is 0.5 mm or less and height of thedistal end of said protrusion decreases by an angle in a range from 10to 60° toward said land surface.
 10. The throw-away tip according toclaim 1, which has a linkage portion that connects at least one of saidprotrusions and the side edge of said land surface.
 11. The throw-awaytip according to claim 10, wherein a length a of one side edge of saidprincipal surface and a distance c between a corner having an angle of90° or less and said linkage portion satisfy a relationship:0.25a≦c≦0.75a.
 12. The throw-away tip according to claim 10, whereinsaid principal surface has diamond shape.
 13. The throw-away tipaccording to claim 10, wherein a length a of one side edge of saidprincipal surface and a width d of said linkage portion satisfy arelationship: 0.5 mm≦d≦(a/3).
 14. A throw-away tip of substantially flatplate shape, comprising two principal surfaces, that provide rake facesand seating surfaces, and side surfaces that provide relief faces, withat least the corner portions of the intersects of the principal surfacesand the relief faces are used as cutting, edges such that the throw-awaytip can be used on both sides, wherein each of said principal surfacecomprises a land surface provided at l ast along the periphery thereofthat forms said cutting edge, a central surface provided on the insideof said land surface, and a recess interposed between the land surfaceand the central surface; when one of said principal surfaces is theseating surface, both the central surface and the land surface of thisprincipal surface serve as the contact surface; and said recess thatcontinues from said land surface in said corner portion is dimensionedin a range from 0.1 mm to 0.6 mm in recess depth (h₁), from 0.7 mm to 2mm in recess width (w), from 0.3 mm to 1.5 mm in width (w₁) from theland surface side to the bottom of the recess, and from 0.5 mm to 1.7 mmin width (w₂) from the bottom of said recess to said central surface.15. The throw-away tip according to claim 14, wherein said recess thatcontinues from said land surface in said corner portion comprises threecurves; a first curve (10″) having a radius of curvature (r₁) in a rangefrom 12 mm to 18 mm, a second curve (11″) having radius of curvature(r₂) in a range from 0.1 mm to 0.6 mm and a third curve (12″) havingradius of curvature (r₃) not smaller than 50 mm, that are connectedsmoothly in this order from aid land surface side.
 16. The throw-awaytip according to claim 14, wherein a linkage portion is provided thatconnects said land surface and said central surface at a position of theperiphery of said principal surface which is not used as a cutting edge,thereby providing a contact surface.
 17. The throw-away tip according toclaim 14, wherein at least one protrusion that extends toward said landsurface is provided on said central surface.
 18. The throw away tipaccording to claim 17, wherein said protrusion is formed to have a sharppoint.
 19. The throw-away tip according to claim 17, wherein one of saidprotrusions extends toward said corner portion.
 20. The throw-away tipaccording to any one of claims 4 through 6, wherein both sides of saidprotrusion have such a configuration as to slope downward and then risetoward said land surface.
 21. The throw-away tip according to claim 14,wherein a ratio (L₂/L₁) of the distance L₂ from the center of saidthrow-away tip to the distal end of said protrusion to the distance L₁from the center of said throw-away tip to the periphery of thethrow-away tip is in a range from 0.7 to 0.95.
 22. The throw-away tipaccording to claim 14, wherein a ratio of areas of said central surfaceand said land surface in said principal surface is in a range from 50 to90%.
 23. The throw-away tip according to claim 14, wherein a width ofsaid land surface is in a range from 0.2 to 0.5 mm.